Vibrating plates for sound instruments and method of manufacturing the same

ABSTRACT

A VIBRATING PLATE FOR SOUND INSTRUMENTS COMPRISING A PLURALITY OF FOAMED GRANULES OF A THERMOPLASTIC RESIN AND A BOUNDARY LAYER OF A THERMOSETTING RESIN ADHERING SAID FOAMED GRANULES TOGETHER TO FORM AN INTEGRAL STRUCTURE.

GENICHI KAWAKAMI OF MANUFACTURING THE SAME Filed Oct. 13, 1969 VIBRATING PLATES FOR SOUND INSTRUMENTS AND METHOD Nov. 9, 1971 3,618,442 VIBRATING PLATES FOR SOUND INSTRUMENTS AND METHOD OF MANUFACTURING THE SAME Genichi Kawakami, Hamakita-shi, Japan, assignor to Nippon Kakki Seizo Kabushiki Kaisha, Hamamatsushi, Shizuoka-ken, Japan Continuation-impart of application Ser. No. 580,803, Sept. 20, 1966. This application Oct. 13, 1969, Ser. No. 866,027 Claims priority, application Japan, Sept. 25, 1965, 40/58,570 Int. Cl. C08j 1/14; G10d 15/00 US. Cl. 84-452 15 Claims ABSTRACT OF THE DISCLOSURE A vibrating plate for sound instruments comprising a plurality of foamed granules of a thermoplastic resin and a boundary layer of a thermosetting resin adhering said foamed granules together to form an integral structure.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of my co-pending United States patent application Ser. No. 580,803, filed Sept. 20, 1966, now abandoned.

SUMMARY OF THE INVENTION This invention relates to improvements in a vibrating plate made of synthetic resin material for use in sound instruments.

More particularly, this invention relates to a vibrating plate having a specific internal structure which is most suitably used as a sounding board of a piano or a guitar, or a diaphragm for the loudspeakers of various electronic musical instruments.

Although loudspeakers for use in high-fidelity sound reproduction systems such as stereophonic sound systems are required to have sound pressure frequency characteristics as wide and as flat as possible, this is not necessarily desirable for a vibrating plate for musical instruments. A sounding board of a piano, for instance, preferably has a multiplicity of resonance characteristics which modify otherwise rather monotonous sound into more vivid and natural sound, rendering the richness and expression desirable for musical instruments. The same thing can be said with regard to loudspeakers for electrical musical instruments.

As is well known in the art, materials having a small specific gravity, a high Youngs modulus, and a relatively great internal loss are desirable in a vibrating plate for sound instruments. T meet these requirements, straightgrained boards of spruce reinforced by sounding bars or ribs have been used in the sounding boards of pianos; papers with various treatments and fabrications have been used for diaphragms of loudspeakers, etc. However, the above materials have not necessarily satisfied the aforementioned requirements.

In recent years, foamed materials of various synthetic resins such as polystyrene, polyvinyl chloride and the like have been used for vibrating plates of sound instruments. The use of such material for the vibrating plates of sound instruments has certain advantages in that an excellent sound radiation efficiency is obtained due to its extremely small specific gravity while eliminating undesirable extraordinary partial resonance of the vibrating plate due to its relatively large internal loss. In addition, it can be molded into any desired shape.

However, on the other hand, when using such material for the vibrating plates of sound instruments, there is a United States Patent 0 3,618,442 Patented Nov. 9, 1971 definite drawback in that the decrease in output level in the frequency range is considerable due to a small Youngs modulus. Unfortunately, there is a close relationship between the specific gravity and the Youngs modulus of such material wherein the Youngs modulus decreases with decreasing specific gravity. Thus, because a high Youngs modulus cannot be obtained along with a loW specific gravity, vibrating plates made of such materials have not been capable of affording satisfactory per formance.

Moreover, foamed materials of synthetic resins known heretofore have mechanical strengths which are insufficient to withstand vibrations and are easily broken internally thereby. As a result, vibrating plates made of such material are not durable.

It is, accordingly, an object of the present invention to provide a vibrating plate for sound instruments affording satisfactory performance Without being accompanied by the disadvantages of the prior art vibrating plates heretofore described.

In accordance with the present invention, a vibrating plate for sound instruments essentially consisting of a plurality of foamed granules of a thermoplastic resin and a boundary layer of a thermosetting resin adhering said foamed granules together to form an integral structure, is provided.

In the vibrating plate of the present invention, since a thermosetting resin which normally has a high Youngs modulus is used for filling voids formed between the foamed granules of a thermoplastic resin having a small Youngs modulus, it is possible to increase the overall Youngs modulus of the vibrating plate as a whole without an accompanying increase in specific gravity. Hence, in accordance with the present invention, there is obtained quite easily a vibrating plate for sound instruments having a high sound-radiating efficiency and a low output deficiency in the high-frequency range. Furthermore, the plastic material can be molded into any desired shape.

More specifically, the primary feature of the present invention resides in providing a vibrating plate which has a cellular structure encased within a denser framework by surrounding the foamed granules of a thermoplastic resin having a relatively small specific gravity with a thermosetting resin material having a relatively high stiffness. The resultant vibrating plate has a light weight and is quite stiff. By the specific internal structure of the vibrating plate wherein the foamed granules of a thermoplastic resin are embraced by a thermosetting resin material of usually high mechanical strength and weathering resist ance, the durability thereof is remarkably superior to that of conventional foamed plastic vibrating plates. Furthermore, excellent performance with regard to frequency characteristics is obtained.

Moreover, when it is necessary for a vibrating plate to have a specific gravity and a Youngs modulus in certain predetermined ranges, such requirements can be easily met by changing the mixing proportion of these two resins and adjusting the degree of foaming of the thermoplastic resin.

Any thermoplastic resin which can be produced in foamed beads may be utilized in the practice of the present invention as long as such resins have a specific gravity of from 0.02 to 0.1 when foamed, and a molecular weight of from 100,000 to 1,000,000; preferably 150,000 to 300,000.

The most preferable ones among such resins are polystyrene and copolymers of styrene/diallylphthalate having more than 70% of styrene component.

Thermoplastic resin which may be used in the present invention includes polyolefins such as polyethylene and polypropylene. In addition, polyvinyl chloride, polyurethane, and acrylonitrile-butadiene-styrene resins that meet the above criteria work well in the practice of the present invention.

In the practice of the present invention, there may be utilized any thermosetting resin which is in the state of liquid of A-stage having, a molecular weight of e.g. from 100 to 1,000, a specific gravity of from 0.8 to 1.4 and a viscosity of less than 8,000 cp. before curing, is curable at a temperature lower than the softening point of the thermoplastic resin of the foamed beads, and is in the state of solid as a stiff and self-supporting body having a specific gravity of from 0.9 to 1.5 and Youngs modulus of from 10 to 10 km./mm. after curing. In addition, it is important that the thermosetting resin per se or attendant ingredients such as, e.g. solvents, catalysts and curing agents, used for the dissolving and the curing thereof are inert to the thermoplastic resins of the foamed beads in the plasticized state. When the thermoplastic resin is polystyrene or styrene/diallylphthalate copolymers, the thermosetting resin is preferably water soluble or alcohol soluble.

The most preferable ones among such thermosetting resins are epoxy resin of bis-phenol A diglycidyl ether type, resorcinol paraformaldehyde resin, phenol formaldehyde resins of resol type and xylene formaldehyde resin, all in pre-polymer state.

Although the use of a curing agent in curing the thermosetting resins of the present invention is not essential, the use thereof is desirable in insuring such curing in a short period of time.

In the present invention, any curing agents which can cure the thermosetting resin used at a temperature lower than the softening point of thermoplastic resin of foamed bead can be used. Such curing agents include, for example, polyethylenepolyamine such as ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine, polymethylenepolyamine such as hexamethylenetetramine, and benzenesulfonic acid.

In the manufacture of the present invention, granules of thermoplastic resin are prefoamed in a convertional manner by heating same with steam at about 100 C. for approximately one minute. Foamed beads of suitable particle size (preferably from 68 mesh ASTM) are then selected and are mixed and kneaded with a binder chosen from the above A stage thermosetting resins. The resultant mixture is then placed into a mold, and is heated at a temperature not exceeding the softening point of the thermoplastic resin in use but which is sufficient to cure the thermosetting resin. A pressure of from to 10 kg./mm. (preferably from 1-5 kg./mm. is simultaneously applied for a. period of time sufficient to cause the thermosetting resin to cure to a point where an integral self-sustaining product is formed. The mixture in the mold is im mediately quenched thereafter, and the resultant product is removed from the mold and allowed to stand for at least hours at room temperature. Alternatively, upon being taken from the mold, the resultant product may be heated at about 50 C. for about 5 hours until the thermosetting resin is completely cured.

The above molding operation may be conducted under atmospheric pressure, and in doing so, it is desirable that the mixture in the mold be first heated at a temperature of about 100 C. for about one minute followed by heating at a temperature slightly higher than 100 C., e.g., 125 C., for a relatively short period of time, e.g., seconds, in order to induce secondary foaming of the prefoamed 'beads as well as to melt the surfaces thereof in order to insure better adhesion between the foamed beads and the boundary layer of the thermosetting resin.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing illustrates, by way of example, an enlarged cross-sectional view of a portion of the vibrating plate of the present invention near the surface of one side thereof.

Referring to the drawing, it can be seen that foamed thermoplastic resin granules 1 are adhered together by boundary layer 2 of a thermosetting resin, to form the present vibrating plate.

The invention is further illustrated in detail by the following examples, which are not intended to be limitative of the scope of the invention.

Example 1 At room temperature, A stage thermosetting binder in liquid state was first prepared by mixing a thermosetting resin called Epikote No. 828 (trade name, product of Shell Chemicals Co., Japanep0xy resin of bisphenol A diglicidyl ether type having molecular weight of about 380) and a curing agent called Thomide No. 235$ (trade name, product of Fuji Kasei Kogyo K.K., Iapantetraethylenepentamine having molecular weight of 700 to 1,000), the weight ratio of the thermosetting resin to the curing agent being 1 to 0.2, the mixture having a viscosity of 7,000 cp. Then 38 g. of polystyrene foamed beads having a molecular weight of 200,000, a specific gravity of 0.04 and a particle size of from 6 to 8 mesh ASTM were put into a kneading vessel and then 20 g. of the above-prepared mixture of the thermosetting resin is poured little by little while kneading.

The resultant mixture was put into a mold having the shape of a vibrating plate, and the mold was heated at C. under a pressure of 3 kg./cm. (gauge) for 30 minutes. At the end of this period, the mold was quenched with water for 6 minutes and the molded product was taken out. This product was then allowed to stand for 24 hours at room temperature.

The resulting vibrating plate had a specific gravity of about 0.1, a Youngs modulus of about 10 kg./mm. and a tensile strength of about 0.2 kg./mm.

Example 2 Example 1 was repeated using the same foamed beads and the same procedures except that 20 g. of a mixture of Epikote No. 828 and Versamid 115 (trade name, product of General Mills-a versamide fatty polyamide resin having molecular weight of 800l,000) was used as a binder. In the binder mixture, the ratio of EpikotezVersamid was 1:1, this mixture having a viscosity of 8,000 cp.

The resultant vibrating plate had identical physical properties to those of the vibrating plate of Example 1.

Example 3 Example 1 was repeated using the same foamed beads and the same procedures described therein except that for the thermosetting resin mixed with curing agent, 20 g. of a mixture of Plyophen No. 6000 (trade name, product of Reichhold Chemicals, Inc.resorcinol/paraformaldehyde resin having molecular weight of -400) and Plyophen Catalyst No. 6002 (trade name, product of Reichhold Chemicals, Inc.hexamethylenetetramine) was used. The weight ratio of thermosetting resin to curing agent was 110.2, resulting in a mixture having a viscosity of 3,000 cp. The molding period used was about 15 minutes.

The resulting vibrating plate had physical properties similar to those of the vibrating plate obtained in Ex ample 1.

Example 4 Example 3 was repeated using the same foamed beads and the same procedures described therein except that the thermosetting resin-curing agent mixture consisted of 20 g. of a mixture of Plyophen No. 5023 (trade name, product of Reichhold Chemicals, Inc.phenol/formaldehyde resin, resol-type, molecular weight l40280) and benzenesulfonic acid. The weight ratio of thermosetting resinccuring agent was 110.1; the resultant mixture having a viscosity of 3,000 cp.

Example 5 Example 3 was further repeated using the same foamed beads and according to the same procedures as described therein, except that as the thermosetting resin-curing agent mixture, 20 g. of a mixture of 15.7 g. of Epikote No. 828 defined in Example 1), 2.5 g. of Tritex H300 (trade name, product of Taiho Kogyo K.K., Japanaliphatic polyamine) and 1.8 g. of methanol, were used.

The resulting vibrating plate had physical properties similarto those of the vibrating plate obtained in Example 1.

Example 6 Example 3 was repeated using the same foamed beads and the same procedures described therein except that Nikanol (trade name, product of Nihon Gas Kagaku Kogyo K.K.m-xylene-formaldehyde prepolymer having a specific gravity of 1.05, a molecular weight of 300 before curing and a viscosity of 4,000 cp. at A stage at 30 C.) was used as a thermosetting binder and 0.05 parts by weight of Nikanol LL (trade name, product of Nihon Gas Kagaku Kogyo K.K.-monoethylenediamine) was used per part by weight of said binder as a catalyst therefor.

The resulting vibrating plate had physical properties similar to those of the vibrating plate obtained in Example 3.

Similar results were obtained when foamed beads of a styrene/diallylphthalate copolymer consisting of 80% by weight styrene and 20% by Weight diallylphthalate was substituted for the polystyrene foamed beads, though more than 70% by weight of styrene may be used with a good result.

In the vibrating plate of the present invention, the weight of thermosetting binder is about 0.1 to 2 parts by weight per part by weight of foamed beads. If less than 0.1 part by weight of binder is used, the mechanical strength of the resulting vibrating plate is too low, and, with the use of more than 2 parts by weight of binder, the weight of the resulting vibrating plate is too high.

I claim:

1. A vibrating plate for sound instruments which is strong, has a high sound radiation efficiency and a low output deficiency in the high frequency range, consisting of foamed granules of a thermoplastic resin having a molecular weight of from 100,000 to 1,000,000 and a specific gravity when foamed of from 0.02 to 0.1 gram per cubic centimeter adhered together in an unsupported integral mass by a boundary layer of a thermosetting resin which before curing is an A stage liquid having a molecular weight of from 100 to 1,000, a specific gravity of from 0.8 to 1.4 and a viscosity of less than 8,000 cp., is curable at a temperature lower than the softening point of the thermoplastic resin of the foamed beads, and after curing having a specific gravity of from 0.9 to 1.5 and Youngs modulus of from to 10 kg./mm.

2. The vibrating plate of claim 1 wherein said thermosetting resin is selected from the group consisting of epoxy formaldehyde, phenol formaldehyde, polyesterformaldehyde, melamine formaldehyde, urea formaldehyde, and xylene-formaldehyde resins.

3. The vibrating plate of claim 1 wherein the thermoplastic resin has a molecular weight of from 150,000 to 300,000.

4. The vibrating plate of claim 1 wherein said thermoplastic resin is selected from the group consisting of polyethylene, polypropylene, polystyrene, polyvinylchloride, polyurethane, and acrylonitrile-butadiene-styrene resins.

5. The vibrating plate of claim 1 wherein said foamed granules of thermoplastic resin have a particle size of from 6 to 8 mesh ASTM.

6. The vibrating plate of claim 1 wherein the weight of said boundary layer of thermosetting resin is about 0.1 to 2 parts by weight per part by weight of said foamed granules of thermoplastic resin.

7. A vibrating plate according to claim 1 which is shaped as the diaphragm of a loudspeaker.

8. A method for making a vibrating plate for sound instruments comprising: forming a mixture consisting of foamed beads of a thermoplastic resin having a molecular weight of from 100,000 to 1,000,000 and a specific gravity of from 0.02 to 0.1 when foamed and a liquid thermosetting resin in the A stage having a molecular weight of from to 1,000, a specific gravity of from 0.8 to 1.4 and a viscosity of less than 8,000 cp., with or without a curing agent; placing said mixture into a mold; heating said mixture in said mold at a temperature not exceeding the softening point of said thermoplastic resin under a pressure of from 0-10 kg./cm. for a period of time sufiicient to cure said thermosetting resin to a point where said mixture forms a solid resultant product; removing said resultant product from said mold; and allowing said resultant product to stand for a period of time sufficient to completely cure said thermosetting resin thereby forming an unsupported shaped article.

9. The method of claim 8 comprising allowing said resultant product to stand for at least 15 hours at room temperature to completely cure said thermosetting resin.

10. The method of claim 8 comprising heating said resultant product at about 50 C. for about 5 hours to completely cure said thermosetting resin.

11. The method of claim 8 further comprising mixing a curing agent selected from the group consisting of ethylene-diamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, hexamethylenetetramine and benzenesulfonic acid with said thermosetting resin prior to mixing same with said foamed beads.

12. The method of claim 8 further comprising quenching said mold to cool same prior to removing said resultant product.

13. The method of claim 8 further comprising initially heating said mixture in the mold at about 100 C. for about 1 minute and directly thereafter heating said mold at a second temperature slightly higher than 100 C. for a second period of time of less than 1 minute in order to induce secondary foaming of said foamed beads and to melt the surfaces of same to insure better adhesion between said foamed beads and said thermosetting resin.

14. The method of claim 13 wherein said second temperature is C. and said second period of time is about 25 seconds.

15. The method of claim 8 comprising utilizing a pressure of from 1 to 5 kg./cm. while said mixture is being heated in said mold.

References Cited UNITED STATES PATENTS 2,862,834 12/1958 Hiler 260-25 B 3,023,136 2/1962 Himmelheber et al. 2'602.5 B 3,395,775 8/1968 Smith 2602.5 Di

JOHN C. BLEUTGE, Primary Examiner M. F OELAK, Assistant Examiner US. Cl. X.R. 

